Masonry thickness around firebox and heat ducting?

I think the minimum masonry thickness around the heat exchange ducts in a rmh,according to the book,is 4 inches.
If a thermal mass has a brick casing,does the thickness of the brick casing counts to the above 4 inches?
Or we just take into account the thermal mass between the casing and the ducts (for example the thickness of cob and stone
between the ducts and the casing?).
What would be the minimum masonry thickness around the firebox?And would the brick casing again count to the above thickness?

My 6 inch rocket had 6 inches of clay perelite around the entire fire box, too get it nice and hot for a good clean burn - after 6 hours of burning  (1100 + F) , the outside brick (another 2 inches thick) would still be at room temp.
The mass was a measured 6 inches of clay/sand from pipe to pipe - then I added another layer of brick for durability (+ another 2 inches or so).

After a full 6 hour burn - working out in the shop - the entire mass would be 100 F - just like a heating pad.

The thinner the mass, the faster it heats up, and the hotter it gets.  My mass was fairly thick, so it would take longer too heat up ( would not even feel a 2 hour burn).  4 inches sounds about right.

edit :  sorry did not answer the question - add all the mass together - just because its brick, its still mass.